1. Field of the Invention
This invention relates to insulators having high electrical resistance and to a method for increasing the conductivity on surfaces of insulators of organic material such as ceramic, glass, etc., or of inorganic materials such as plastic, epoxy resin, other casting resins, etc., or of mixtures of both materials.
2. Description of the Prior Art
Insulators of organic or inorganic materials such as ceramic, glass, plastic and resin are used for separating conductors so as to prevent undesired flow of current from the conductors to other objects. These insulators have high electrical resistance, usually above 12 ohm cm and normally exhibit a specific bulk resistance of about 10.sup.14 ohm cm. As a result leakage currents come about which, while negligible in normal cases, lead to surface charges, as for instance, by the presence of irregularities on the surface of the insulator. The leakage currents are in the nanoampere range, for a resistivity of about 10.sup.14 ohm cm and 300 kV. Such irregularities may be due to the material; they can also be due to cleaning agent residues which remained on the surface of the insulator from cleaning.
These uneven voltage distributions on the surface of the insulator can later cause discharges, which under some circumstances can bring about the destruction of the insulator. One strives here either to avoid the generation of local charges altogether or to make provision that the charges can flow off.
In outdoor support insulators, the outside surface of the insulator had been coated with a semiconducting layer in order to conduct off charges which, due to soiling of the surface, have settled on the outside surface of the insulator. Such semiconducting layers are, for example, graphite or the like. With the presently known methods of application, the resistance of such semiconducting layers is too low; it is in the range of about 10.sup.4 to 10.sup.5 ohm. In applications of high-voltage d-c transmission (EHV), with the voltage differences present between the inner and outer conductor, this leads to currents which excessively heat-up the insulator, making the latter no longer usable.
Other materials, such as metals which are applied by known methods, e.g., plasma spraying, physical or chemical vapor deposition sputtering, electrolysis, electrophoresis, reactive vapor deposition or ion plating as well as carbonizing or irradiation with alpha-, beta- or gamma rays etc., are disadvantageous even if they are applied to the insulator in only atomic thicknesses, as too large a current flows. In addition, the quality of the layer deposited on the insulator is not sufficiently high, in part because it is difficult and possible only at great cost to fix the layer thickness accurately, and partly because the adhesion of the layer to the insulator is poor. Further, there is danger of radiation damage and, in the case of the use of organic materials, of carbon formation.